1) Field of the Invention
The present invention relates to a semiconductor laser module used for an optical communication apparatus, and more specifically relates to a laser module for optical signal transmission or for a pump light source used for a wavelength division multiplexing system (WDM).
2) Description of the Related Art
Semiconductor laser devices can obtain a high laser output power by increasing an injected current, but the heat output from the device itself generally increases, in proportion to the injected current. The increased heat affects the properties of the semiconductor layer or optical parts which constitute the semiconductor laser device, causing various problems such that the wavelength of the laser actually output is deviated from a desired wavelength or the life of the device is shortened.
Particularly, in a semiconductor laser device used for dense WDM, it is required that the wavelength of the optical signal is stable for a long period of time, and hence it is necessary to accurately perform wavelength control. Therefore, a technique of providing a wavelength monitoring function in a laser module with the semiconductor laser device embedded therein has been well known.
FIG. 28 is a sectional side view of a conventional laser module in a laser outgoing direction. In FIG. 28, in a conventional laser module 300, a ferrule 12 for holding an optical fiber 11 is provided at an opening of a package 101, that is, in a light outgoing portion. On the bottom of the package 101, a first thermo-module 68 and a second thermo-module 69 are arranged close to each other. The first thermo-module 68 and the second thermo-module 69 are apparatus, the surface of which can be heated or cooled depending on the size and direction of the current to be passed, and are formed of a Peltier element or the like.
A base 30 formed of CuW or the like is arranged on the first thermo-module 68. On the top of the base, a submount 34 on which a semiconductor laser device 20 is mounted, a focusing lens 33 which focuses laser beams output from the front end face of the semiconductor laser device 20 onto an optical fiber 11, an optical isolator 32 which interrupts reflected return light from the optical fiber 11 side, and a collimator lens 35 which collimates the monitoring laser beams output from the rear end face of the semiconductor laser device 20, are provided. The portion including the base 30, the focusing lens 33, the submount 34, and the collimator lens 35 is referred to as a laser section.
On the other hand, a base 50 formed of CuW or the like is put on the second thermo-module 69, and on the top of the base, a prism 51 that splits the monitoring laser beams output from the rear end face of the semiconductor laser device 20 into two directions at a predetermined angle, an optical filter 52 to which one of the beams split by the prism 51 enters, and a submount 53, are provided. On the front face (a face in the laser outgoing direction) of the submount 53, a first optical detector 41 which receives the other of the beams split by the prism 51, and a second optical detector 42 which receives the beam passing through the optical filter 52 are provided on the same plane of the submount. A photo diode is used for the first optical detector 41 and the second optical detector 42.
A thermistor 54 that monitors the temperature of the optical filter 52 is provided near a portion where the prism 51 is fixed. The portion including the base 50 and each component provided on the base 50 is referred to as a wavelength monitoring section.
In this laser module 300 having the configuration, stable laser emission is realized by controlling the temperature of the first thermo-module 68 and the second thermo-module 69. The temperature control in this laser module 300 will be briefly explained below. The monitoring laser beam output from the rear end face of the semiconductor laser device 20 passes through the collimator lens 35, and the beam is split into two directions by the prism 51.
The one of the beams split by the prism 51 is converted into electric current by the first optical detector 41, and is used as a reference voltage in a not-shown current-voltage converter. The other of the beams split by the prism 51 passes through the optical filter 52, and the beam is converted into electric current by the second optical detector 42, and is used as a signal voltage in the not-shown current-voltage converter. The optical filter 52 has a property such that the transmission factor thereof is different with respect to the wavelength of the incident beams, and is formed of for example etalon. Therefore, when it is assumed that a difference between the signal voltage obtained with beams having a desired wavelength passing through the optical filter 52 and the reference voltage is a standard voltage difference, a wavelength deviation can be found by comparing the voltage difference between the actual reference voltage and the signal voltage with the standard voltage difference.
Since the wavelength deviation can be corrected by changing the temperature of the semiconductor laser device 20, the temperature of the submount 34 located below the semiconductor laser device 20 may be adjusted (cooled or heated) in order to correct the deviation. Therefore, a not-shown controller uses the voltage indicating the wavelength deviation obtained by the comparison as a control voltage for controlling the temperature of the first thermo-module 68, to operate the first thermo-module 68 as a temperature adjuster. As a result, the semiconductor laser device 20 is feedback controlled so that the temperature thereof is adjusted via the first thermo-module 68, the base 30, and the submount 34, to thereby suppress changes in the wavelength. That is, laser beams having a desired wavelength are output (hereinafter, this controlled state is referred to as wavelength locking).
However, since the optical filter 52 formed of etalon changes the property depending on the temperature, it is desirable to keep the temperature constant. Therefore, the not-shown controller calculates a difference between a desired temperature and the temperature detected by the thermistor 54, to control the temperature of the second thermo-module 69, designating the voltage corresponding to the difference as a control voltage. As a result, the optical filter 52 is heated or cooled via the second thermo-module 69 and the base 50, and stabilized at a desired temperature.
In the conventional laser module, however, since the temperature of the semiconductor laser device 20 is controlled by only the first thermo-module 68, there is a problem that a wavelength variable range, that is, a temperature variable range is not sufficient for realizing a so-called wavelength variable type laser module that selects the temperature of the semiconductor laser device 20 within a predetermined range and uses a laser beam having a wavelength emitted at the selected temperature. As the cause thereof, it can be considered that the cooling ability of the thermo-module unit is not sufficient, and the temperature of the package becomes high due to heat transmitted from the thermo-module.
Insufficient cooling ability of the thermo-module unit will be first explained. The temperature range that can be controlled in the normal thermo-module is about 60xc2x0 C., and therefore, when temperature of from xe2x88x925xc2x0 C. to 70xc2x0 C. are required as the temperature specification of the laser module package, the temperature variable range by the first thermo-module 68 becomes from 10xc2x0 C. to 55xc2x0 C., and hence it is possible to adjust the semiconductor laser device 20 in a range of about 45xc2x0 C. The temperature dependency of the emission wavelength of the semiconductor laser device is determined by the material of the semiconductor, and it is known that the temperature dependency thereof is about 0.1 nm/xc2x0 C. Therefore, in this example, the wavelength variable range becomes 0.1 nm/xc2x0 C.xc3x9745xc2x0 C.=4.5 nm. In this wavelength variable range, however, it is difficult to correspond to various applications having different emission wavelengths that are desired to use, and hence the practicality thereof is poor.
The problem that the temperature of the package becomes high will now be explained. In the thermo-module formed of an electric heat converter such as a Peltier device, heat transfer is realized only between the upper surface where the semiconductor laser device is mounted and the lower face. Therefore, sufficient cooling of the semiconductor laser device results in rise of the temperature on the lower face. Since the lower face of the thermo-module abuts against the bottom face of the package, the temperature rise thereof causes a temperature rise of the package. That is to say, the temperature inside the package increases due to the heat of the package itself, in addition to heat generation of a member to be cooled such as the semiconductor laser device. In the end, within the package of the laser module, such a heat cycle occurs that heat flows into the semiconductor laser device continuously, due to a convection current, radiation, and conduction (of these, mainly due to convection current). The thermo-module must absorb the heat continuously in order to keep the member to be cooled at a predetermined temperature. Therefore, the temperature range that can be actually controlled by the thermo-module becomes narrower than the temperature variable range that can be controlled by the original ability of the thermo-module.
Particularly, since the member to be cooled includes not only the semiconductor laser device but also optical members on the wavelength monitoring section, a thermo-module having a large cooling ability is required. The thermo-module having a larger cooling ability naturally produces a larger amount of heat on the lower face thereof when the cooling ability is exhibited to the maximum, and hence the amount of heat flowing into the semiconductor laser device from the package also increases. For example, when a heating value of the semiconductor laser device is about 0.1 W, the heat flowing into the member to be cooled from the package may exceed 1 W.
As a temperature difference between the package and the member to be cooled increases, heat flowing into the member to be cooled from the package also increases. Therefore, in the wavelength variable type laser module, if it is tried to expand the temperature control range of the semiconductor laser device towards the low temperature side in order to expand the wavelength variable range, the amount of heat flowing therein from the package increases many times as large as the heating value of the semiconductor laser device itself. Hence, the amount of heat to be absorbed by the thermo-module rapidly increases. Since the amount of heat that can be absorbed by the thermo-module is determined by the cooling ability of the thermo-module, the temperature range of the semiconductor laser device that can be controlled is inevitably limited, thereby causing a problem in that the wavelength variable range cannot be expanded. In the state that the temperature difference occurs, the power of the thermo-module required for cooling also increases.
When the heating value of the package, that is, the heating value of the laser module increases, heat dissipation to the air outside the package becomes difficult. This heat dissipation can be realized to some extent by providing a large fin for heat radiation in the laser module or equipment for optical communication including the laser module. However, existence of the large fin causes a new problem that the degree of integration of the laser module and other equipment is decreased.
It takes a long period of time of from tens of seconds to several minutes, until the semiconductor laser device becomes stable at a target temperature set by the thermo-module, mainly because the heat capacity of the member to be cooled is large. Hence, there is a problem that it is difficult to shift to stable emission operation within a short period of time, after activation of the laser module or after the wavelength is changed.
It is an object of the present invention to provide a laser module in which stable wavelength locking is possible by a wavelength monitor using an optical filter, and in which the variable range of an emission wavelength can be expanded by expanding the temperature control range of a semiconductor laser device.
It is another object of the present invention to provide a laser module that realizes expansion of the temperature controllable range of the semiconductor laser device, that is, expansion of the variable range of emission wavelength, as well as realizing low power consumption of a thermo-module and easy heat radiation, and also allows high-density packaging.
It is still another object of the present invention to provide a laser module that can shorten the time until reaching the stable operation, after activation of the laser module or after the wavelength is changed.
The laser module according to the present invention comprises a laser module having a first temperature adjuster and a second temperature adjuster provided on the first temperature adjuster, and also comprises a semiconductor laser device provided on the second temperature adjuster, and a wavelength monitoring section that is provided on the first temperature adjuster and detects a change in wavelength of a laser beam output from the semiconductor laser device. According to this invention, the wavelength monitoring section is kept at a constant temperature to thereby stabilize the wavelength discrimination characteristic of various parts constituting the wavelength monitoring section, and the second temperature adjuster that controls the temperature of the laser section, is provided on the first temperature adjuster that is controlled at a constant temperature. As a result, the temperature variable range of the laser section can be expanded.
The laser module according to the present invention comprises a first temperature adjuster, a semiconductor laser device provided on the first temperature adjuster, a second temperature adjuster that heats the semiconductor laser device at a near place thereof, and a wavelength monitoring section that is provided on the first temperature adjuster and detects a change in wavelength of laser beams output from the semiconductor laser device. According to this invention, the semiconductor laser device can be heated, separately from the first temperature adjuster, by the second temperature adjuster such as a heater.
The laser module according to the present invention comprises a first temperature adjuster, a second temperature adjuster provided on the first temperature adjuster, a third temperature adjuster arranged side by side in isolation from the first temperature adjuster, a semiconductor laser device provided on the second temperature adjuster, and a wavelength monitoring section that is provided on the third temperature adjuster and detects a change in wavelength of a laser beam output from the semiconductor laser device. According to this invention, the wavelength monitoring section is kept at a constant temperature by the third temperature adjuster to thereby stabilize the wavelength discrimination characteristic of various parts constituting the wavelength monitoring section, and the second temperature adjuster that controls the temperature of the laser section is provided on the first temperature adjuster controlled at a constant temperature. As a result, the temperature variable range of the laser section can be expanded.
The laser module according to the present invention comprises a first temperature adjuster, a thermal conductor having high thermal conductivity provided on the first temperature adjuster, a second temperature adjuster provided on the thermal conductor, and a semiconductor laser device provided on the second temperature adjuster. According to this invention, the heat generated on the lower face of the second temperature adjuster for adjusting the temperature of the semiconductor laser device can be efficiently radiated over the whole surface of the first temperature adjuster, by the thermal conductor having high thermal conductivity.
These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings.